1. Field of the Invention
The present invention relates to a noise reduction method, a noise reduction program, a recording medium having the noise reduction program recorded thereon, and a noise reduction apparatus. More particularly, the present invention is applicable to a recursive noise filter for reduction of any noise in a video signal. The present invention relates to mixing of a motion-compensated reference video signal with a non-motion-compensated reference video signal to generate a reference video signal and use of the reference video signal to generate a differential signal in order to sufficiently reduce even a higher noise level.
2. Description of the Related Art
A typical recursive noise filter in related art uses a difference in time between fields or between frames to reduce any noise in a video signal, as shown in FIG. 16. A noise filter 1 shown in FIG. 16 is a motion adaptive noise filter. In the noise filter 1, a delay signal generating unit 2 delays an output video signal S2 for a time corresponding to one field or one frame to supply a reference video signal S3 for extraction of noise components to a subtractor circuit 3. The subtractor circuit 3 subtracts the reference video signal S3 from an input video signal S1 to generate a differential signal S4. A noise extracting unit 4 sets a feedback ratio to a higher value, for example, if the differential signal S4 has a lower amplitude (is likely to be a noise) and sets the feedback ratio to a lower value if the differential signal S4 has a higher amplitude (is not likely to be a noise).
Specifically, the noise extracting unit 4 performs nonlinear processing to the differential signal S4 with, for example, an input-output characteristic shown in FIG. 17 to generate a noise reduction signal S5. A subtractor circuit 5 subtracts the noise reduction signal S5 from the input video signal S1 to generate the output video signal S2.
In contrast, a noise filter 11 shown in FIG. 18 is a motion compensation noise filter. The same reference numerals are used in the noise filter 11 to identify the same components in the noise filter 1 shown in FIG. 16. A description of such components is omitted herein. In the noise filter 11, a motion vector detecting unit 12 detects a motion vector MV from the reference video signal S3 on the basis of the input video signal S1. A motion compensating unit 13 performs motion compensation to the reference video signal S3 with the motion vector MV detected by the motion vector detecting unit 12 to generate a motion-compensated reference video signal S6. The noise filter 11 subtracts the motion-compensated reference video signal S6 from the input video signal S1 to generate the differential signal S4.
For example, Japanese Unexamined Patent Application Publication No. 2001-136416 discloses a method of measuring a noise level and automatically setting a signal level of the noise reduction signal S5 on the basis of the measurement result in such a recursive noise filter.
Generally, it is possible for the motion compensation noise filter to sufficiently reduce the noise level even in motion pictures. However, the motion compensation noise filter can erroneously detect a motion vector when the noise level increases. In this case, it becomes difficult for the motion compensation noise filter to correctly perform the motion compensation to the reference video signal S3, thus preventing the noise from being sufficiently reduced. In addition, the noise reduction level can locally vary in a spatial direction and in a time direction particularly in flat areas. As a result, the noise shows undulation on a screen to cause a user to feel discomfort.
If the noise level increases, the size and direction of the motion vector vary in the spatial and time directions in the flat areas and the spatial and time variations depend on an algorithm of detecting the motion vector. As a result, the motion-compensated reference video signal has any correlation with the input reference signal in terms of the noise and the noise shows the undulation on the screen.